The present invention relates to low power oscillators, and has particular relation to low power oscillators which must be turned on quickly.
FIG. 1 shows a Pierce oscillator. An input node XTALin and an output node XTALout are separated by an inverter, a crystal XTAL, and a resistor R, in parallel. The nodes are typically pads on an integrated circuit. The inverter comprises a PFET in series with an NFET N0. The gates of both FETs are connected to XTALin. The source of P0 is connected to Vcc. The drain of P0 and the source of N0 are connected to XTALout. The drain of N0 is grounded. An input capacitor Cin and an output capacitor Cout provide a load to the crystal XTAL, and are necessary for parallel resonance.
The power dissipated by this oscillator is proportional to the frequency of operation, the capacitance of Cin and Cout, and the square of the voltage across the crystal XTAL. It is often desired to keep this power to a minimum. The first two components are fixed: the frequency is set by the requirements of the device to be driven by the oscillator, and the capacitance is set by the requirements of the crystal. A design opportunity is presented by the third component, however: the voltage used to drive the inverter, typically 5.0 volts, is more than the 2.5 volts typically needed to maintain crystal oscillations. Power consumption could be cut by a factor of four, perhaps from 40.0 milliwatts to 10.0 milliwatts, if the crystal's voltage could be appropriately reduced.
However, starting up the crystal takes 15.0 to 30.0 milliseconds at 2.5 volts, if its starts up at all. Many applications require a start up of 1.0 to 2.0 milliseconds. A modem, for example, may transmit or receive data infrequently. Keystrokes on a keyboard may be separated by 250.0 milliseconds even when a fast typist is operating the device, and even though each key is depressed for only 5.0 to 10.0 milliseconds at a time. It makes no sense to keep the oscillator on when no key is depressed. However, waiting to turn on the oscillator, even if it can be done in the minimum 15.0 milliseconds, will lose the keystroke, even if it has the maximum 10.0 millisecond duration. Continuous 5.0 volt (40.0 milliwatt) operation of the crystal has therefore been the art's necessary, but unsatisfactory, response.